1. Field of the Invention
An apparatus and method for alternately coupling a normal power supply and an emergency power supply to a lighting device. More specifically, the present invention relates to an apparatus and method which uses a DC relay to apply an emergency voltage to a lighting device when a normal voltage falls below a predetermined level.
2. Description of the Related Art
Many businesses and establishments will employ an emergency power supply which can be relied upon when the normal power supply malfunctions. Typically, an emergency power supply is used to supply power to lights in the event that a failure occurs in the normal power supply.
Different types of lighting devices, such as fluorescent and incandescent lights, are used in establishments throughout the world. Other types of high intensity lights, commonly known as high intensity discharge lamps (HID lamps), are used in areas requiring much light, such as in stadiums, parking lots and the like.
Due to the nature in which these types of high intensity lights are used, it is critical that the lamps can be quickly restarted after being shut off intentionally or, for example, due to a power failure. Hence, lamp starting circuits, such as those described in U.S. Pat. Nos. 5,047,694 and 4,890,041 to Nuckolls et al., have been developed which facilitate rapid starting and restarting of high intensity lamps. The entire contents of U.S. Pat. Nos. 5,047,694 and 4,890,041 are incorporated herein by reference.
It is also important to provide an emergency power supply, which will operate in place of the normal power supply if the normal power supply fails, to thus keep the light from turning off unintentionally. Various types of power failure detection circuits are known in the art which can detect when a failure occurs in a main power supply, and then enable power to be provided by an emergency power supply during a failure. However, many of these types of devices are complicated in both design and operation. Furthermore, these devices are also susceptible to minor fluctuations in the voltage provided by the main power supply.
Examples of known emergency lighting circuits are described in U.S. Pat. No. 4,057,750 to Elms et al., U.S. Pat. No. 4,686,424, and published Japanese Patent Application No. 3-49188.
Therefore, a continuing need exists for a device that is capable of effectively providing power from an emergency power supply to a lighting device when a failure occurs in the main power supply, and which is also simple in construction and not adversely affected by fluctuations in the voltage provided by the main power supply.